The ~= sign means “approximately equals to”, and depends on the number of decimals; for example,

~=1.3

in a cell will work for any value between 1.2 and 1.4.

A range allows Slim to accept results between boundaries. For example, put something like

1<_<8

in a cell and all values between 1 and 8 will be considered valid. This is something that some versions of Fit could do, but not all. With Slim, all versions of Slim will support this (since it will be evaluated on the Fitnesse side).

Last thing: Fitnesse contains a rather powerful expression evaluator that is not specific to Slim or Fit.
For example:

!define TIMES {1000}
|${=5*${TIMES}=}|

You can combine this construct with the range, and have something like this in your cell: